The present invention relates to a semiconductor device having an output buffer whose impedance is adjustable.
In recent years, the speed of data communication between semiconductor devices is increasing, so that reflection noise occurring at the time of high-speed communication has to be reduced. Impedance adjusting methods for adjusting the impedance of an output buffer with respect to a resistance value of an external resistor are being developed.
For example, patent document 1 (Japanese Unexamined Patent Application Publication No. 2000-183718) discloses a method of adjusting the impedance of an output buffer to the resistance value of an external resistor.
In FIG. 1 of the patent document 1, the impedance of a pull-down output buffer is adjusted to an integral multiple of the resistance value of an external resistor using information when the pull-down counter adjusts the total channel width to the resistance value of the external resistor by on/off control of a plurality of transistors configuring the impedance of a pull-down dummy buffer. Using information when a pull-up counter adjusted the impedance of a pull-up dummy buffer, the impedance of the pull-up output buffer is adjusted to an integral multiple of the resistance value of the external resistor. For the adjustment of the impedance of the pull-up dummy buffer by the pull-up counter, information of the operation of adjusting the pull-down counter is used.
For example, patent document 2 (Japanese Unexamined Patent Application Publication No. 2006-319968) discloses an impedance control circuit and its control method capable of reducing resistance mismatch between a pull-up device and a pull-down device.
Concretely, to compensate a quantizing error in pull-up and pull-down control code data, by providing a compensation unit in an output driver, resistance mismatch between the pull-up device and the pull-down device is reduced.